A conventional moisture-permeable waterproof sheet is mainly produced by coating a fabric with a solution of polyurethane in a water-soluble solvent, and wet coagulating the polyurethane. Rain or other types of water cannot permeate the porous polyurethane film formed thereon when the solvent is removed with water, but moisture (water vapor) can permeate it.
However, when the porosity of the waterproof fabric is increased (the number of pores are increased and the pore size becomes large) to improve the permeability, the water pressure resistance thereof does not fail to fall, and the waterproof sheet is not waterproof. Conversely, when the water pressure resistance thereof is improved (the number of pores is decreased and the pore size becomes small), the moisture permeability thereof is lowered. Accordingly, the improvement of the moisture permeability conflicts with that of the water pressure resistance.